The present invention is directed to the art of RF broadcast transmission systems and, more particularly, to improvements in providing pre-correction of an information signal prior to amplification and transmission.
It is known that various communications systems require amplification of an information signal. The signal may be provided in an analog format or digital format and converted to an analog signal which is then applied to a high power amplifier.
An amplifier may be modeled as a simple gain block with constant gain. In reality, devices employed for amplifiers include bi-polar transistors, field effect transistors and the like. These devices typically do not have inherent input and output impedance that matches the desired source and load impedance. Matching circuits are employed to transform the amplifying device impedances to those of the desired source and load. These matching circuits do not have infinite bandwidth and therefore create linear distortions (sometimes referred to herein as memory-full distortions).
Amplifiers also create non-linear distortions (sometimes referred to herein as non-memory-full distortions). Fixed supply voltages ultimately limit the maximum available output power from an amplifier and determine the point of amplifier compression. Their mode of operation determines other potential distortions. For example, class A/B or class B amplifiers generate cross-over distortion.
In order to reduce amplifier memory-full and non-memory-full distortions, pre-correction circuits have been employed. Linear pre-correction circuits behave in such a way as to modify a signal input in time or frequency domain. Non-linear pre-correction circuits modify a signal in the amplitude or phase domain.
A transmitter amplifier may be modeled as a cascade of blocks consisting of linear and non-linear blocks. For a typical amplifier, these consist of a high power amplifier input network which has a filter, the amplifying device itself, and an amplifier output network which has a high power output filter. The distortion correction circuits may be arranged in inverse or reverse order in terms of pre-correction linear and non-linear distortion correcting circuits.
Reference is made to a co-pending U.S. patent application Ser. No. 09/312,354 filed on May 14, 1999 entitled BROADCAST TRANSMISSION SYSTEM WITH DISTRIBUTED CORRECTION and which was filed in the names of Edwin R. Twitchell and Robert J. Plonka and assigned to the same assignee as the present application and which co-pending application is herein incorporated by reference. FIGS. 2 and 3 herein are based on that application and each includes a power amplifier having an input filter and an output filter with the filters providing linear distortion to the information signal. The information signal is taken from a data stream that is supplied to pre-correctors located upstream from the power amplifier. The pre-correctors include a pair of linear equalizers and a non-linear corrector.
In accordance with the present invention there is provided a radio frequency (RF) transmitter system having an input circuit for receiving an RF signal to be amplified and transmitted. A series circuit is provided which includes first and second cascaded amplifier networks located downstream from the input circuit and wherein the first amplifier network is located upstream from the second amplifier network and wherein the first and second networks respectively include first and second non-linear amplifiers that exhibit non-linear response characteristics to the RF signal and that introduce non-linear distortions into the RF signal. The second network includes a filter that introduces linear distortions into the RF signal. A first correction circuit is located upstream from the series circuit for pre-correcting the RF signal in a manner to compensate for the linear and non-linear distortions introduced by the second amplifier network. A second pre-correction circuit is interposed between the first correction circuit and the first amplifier network for pre-correcting the RF signal to compensate for the non-linear distortions introduced by the first amplifier network. The second pre-correction circuit includes a third non-linear amplifier selected so as to exhibit a non-linear response characteristic to an applied RF signal similar to that of the first amplifier.